The colonial morphology of the T-mycoplasmas may be rough, fried egg or smooth depending on the constituents of the agar medium. Therefore, their ability to grow in semisynthetic media and their metabolic requirements are being investigated. While T-mycoplasmas are commensals of the urogenital tract, some may be pathogens. In the expectation that characteristic differences may be found the biological properties and resistance to physical stress of 5 laboratory strains and of 5 isolates from 5 patients with non- specific urethritis are being compared. Classical mycoplasmas are being isolated and screened for markers which may be employed in genetic studies.